[Purposes] To overcome the limitations of conventional grouting materials for lining void repair—particularly their high cost and long-term strength retrogression—a novel rapid grouting material was developed. This material is based on an organic–inorganic composite system, in which waterborne epoxy resin is combined with fly ash and silica fume to modify magnesium phosphate cement. [Methods] This study provides a systematic investigation into the effects of the magnesium-to-phosphate ratio (M/P) and the water-to-cement ratio (W/C) on the mechanical properties of WER-modified magnesium phosphate cement? paste. In addition, fly ash and silica fume were incorporated as partial replacements for magnesium oxide to reduce the overall material cost. [Findings] The results indicate that the M/P ratio exhibits similar trends in its influence on both flexural and compressive strengths. An M/P ratio of 3.0 produced the highest early-age strength, with 3-h compressive and flexural strengths reaching 13.4 MPa and 3.8 MPa, respectively. In contrast, an M/P ratio of 2.5 yielded the highest long-term strength, achieving 28-day compressive and flexural strengths of 42.7 MPa and 10.9 MPa, respectively. The effect of the W/C ratio was more straightforward: increasing W/C led to reduced strength at all curing ages. Among the FA/SF combinations, a ratio of 2:1 exhibited superior mechanical performance compared with 3:1. At a total replacement level of 35%, the compressive and flexural strengths reached approximately 45 MPa and 11.2 MPa. [Conclusions] This study provides an efficient and cost-effective grouting material, along with a practical application strategy, for the remediation of tunnel voids. It also offers a theoretical basis for optimizing the mix design and enhancing the modification of magnesium phosphate cement materials.